This invention relates to apparatus for determining roll density in a paper mill, or more particularly for determining roll density in a paper winder or paper winding means used in a paper mill or the like.
Paper roll density (which for most purposes may be assumed to be proportional to roll hardness, at least over the range of interest) is a parameter of considerable interest in the winding of a roll. The density of the outermost layers are of particular interest in that they influence the structure of the completed roll. The density of the outer layers as they are added to the roll during winding is not the same as the density of the finished roll because as the winding progresses the previous outer layers are covered and there is radial pressure now added by the new outer layers.
A number of factors affect roll density and these factors include radial internal pressure (which is difficult to measure directly), web tension, rider roll pressure and drum torque. By monitoring roll density continuously during winding, it is possible to detect errors in the build-up of the roll and to correct the errors or at least be aware of the errors to ensure that unsatisfactory rolls are not shipped.
Various means have been developed to monitor roll density. One such means determines the volume of paper added to the roll by counting the number of revolutions of the paper roll since the last determination and using this with a measurement of paper length to obtain roll diameter. Then roll diameter, or certain of the measurements used to obtain roll diameter, and paper length are used to obtain density. An article entitled "Measurement of Paper Roll Density During Winding" by L. G. Eriksson et al, TAPPI Journal, January, 1983, pages 63-66, describes one form of such a manner of determining roll density.
The determination of roll density is better understood by reference to FIG. 1 which shows one form of prior art apparatus for determing roll density. In FIG. 1, a first and a second drum 10 and 11 support a finished paper roll 12. A web of paper 13 passes under drum 10 and is wound on roll 12. Pulse transmitters 14 and 15 are on drum 10 and roll 12 respectively, and they transmit pulses on conductors 16 and 17 respectively which are proportional to rotation. For example, the number of pulses from transmitter 14 might be of the order of 5000 ppr (pulses per revolution) and will be designated Z hereinafter, and the number of pulses from transmitter 15 might be of the order of one ppr and will be designated V hereinafter. The diameter of roll 12 increases as paper is wound on it. Because the rate of feeding the paper onto roll 12 is substantially constant, that is the speed of drum 10 is approximately constant, the rate of rotation of roll 12 decreases as its diameter increases.
Conductor 17 is connected to a first counter 18 which has set into it at 20 a count N which is the number of pulses received on conductor 17 before counter 18 outputs a pulse on conductor 21. For example N might be set to a value of 50 and if V=1 then there would be 50 revolutions of roll 12 for each pulse on conductor 21. Similarly conductor 16 is connected to a second counter 22 which counts pulses from transmitter 14 and provides an output on conductor 23. The count on conductor 23 represents paper length L.
A number of values are set into the apparatus at input 24. These values include the previously referred to Z and N, and also
m=paper basis weight, that is the weight of paper per unit area PA1 D.sub.D =diameter of the first drum 10.
The count of pulses on conductor 21 may be represented by K and the count is set to one, i.e. K=1 is preset in block 24 before starting. The paper length L is also initialized in block 24 before starting by setting L(K)=0, that is by setting the paper length to zero for the lnitial reading or initial count K=1.
A start circuitry 26 provides a signal on conductor 27 when it receives a pulse on conductor 21. This starts the monitoring apparatus. The start circuitry 26 also includes a counter which keeps track of the number of pulses received from conductor 21. Each pulse increments K by one.
Conductor 27 is connected to read pulse counter block 28. Each time the read pulse counter block 28 receives a signal from conductor 27 it reads the count from counter 22 and stores the value. Thus it has the values for the length at each reading K, i.e. it has L(K), L(K+1) and so on. The read pulse counter 28 provides a signal on conductor 30 representing the count which in turn represents paper length at the current reading K. A decision circuit 31 receives the signal on conductor 30 and determines if K is equal to or greater than three. If not, it provides a signal on conductor 32 back to the start block 26. The purpose of this is to provide values for averaging. At least two pulses on conductor 21 are required to initiate a calculation. In this case two sets of readings are available for averaging.
When decision circuit 31 receives a signal on conductor 30 and the value of K is equal to or greater than three, it provides a signal to the calculate roll diameter circuitry 33. While roll diameter is not as difficult to determine as roll density (and in the prior art it has been determined by measurement as well as by calculation), it is a convenient value to know. For example, a plot of roll density vs. roll diameter provides a useful display to an operator during winding or provides a useful graph for analysis. In addition many of the values required to calculate roll diameter are required to calculate roll density. It will also be apparent that two successive roll diameter determinations can be subtracted to give a diameter difference, and if the number of turns are known a thickness of the paper can be calculated. Knowing paper length and paper basis weight, enables a determination of roll density to be made. The calculate roll density circuitry 34 receives a signal from calculate roll diameter circuitry 33 and initiates a density determination. Signals representing both the determined value for roll diameter and for roll density are provided on conductor 35 which is connected to output results circuit 36.
In this prior art apparatus, the initiation of each reading and the calculations which are initiated depend on the pulses on conductor 21. These pulses depend on a fixed number of revolutions made by the paper roll 12. Thus, the pulses are relatively rapid when the winding starts because the diameter is small and the roll rotates relatively rapidly. As the diameter of the roll increases, its rate of rotation decreases and the time between readings increases. However, there is more paper wound on the roll for each revolution of the roll because of the increasing diameter. Consequently, the actual time between each determination of density increases as the roll of paper is wound, and a greater amount of paper is added to the roll. Thus, as the roll gets longer, the opportunities to make corrections between calculations become farther apart. This is undesirable.